Though some folks have been able to purchase Borderlands early, 2K Games
confirms that the game's DRM will not unlock until next week's scheduled release
dates for Gearbox's role-playing shooter (thanks nin via
IGN).
The 2K
Forums have a comment from 2K community manager 2K Elizabeth explaining the situation:

Yesterday we learned that a fraction of the PC copies of Borderlands were
accidentally sold to consumers in some countries but that gamers could not
activate their copies yet. Since we learned of this problem, we have been
looking into the issue to find the best possible solution for everyone.

I'm going to be completely honest with you guys right now. It is not possible
for us to move the planned release date of the game forward. We could not
possibly get the games into stores worldwide and out for digital distribution
any earlier than planned. Less than 1% of all copies were sold, and if we
unlocked the game today, illegal versions would most likely appear on torrents
by the evening. I am sure all of you want our PC launch to be as successful as
possible, and we simply won't be able to manage that any earlier than already
planned.

I want to apologize for those of you who have gotten the game ahead of our
release date, and ask for your patience and understanding. The game will be
ready in the US and Canada at midnight eastern standard time on Monday, October
26th, and internationally by Friday, October 30th.